Disruption and overexpression of auxin response factor 8 gene of Arabidopsis affect hypocotyl elongation and root growth habit, indicating its possible involvement in auxin homeostasis in light condition

Abstract

SummaryAuxin response factor (ARF) family genes play a central role in controlling sensitivity to the plant hormone auxin. We characterized the function of ARF8 in Arabidopsis by investigating a T-DNA insertion line (arf8-1) and overexpression lines (ARF8 OX) of ARF8. arf8-1 showed a long-hypocotyl phenotype in either white, blue, red or far-red light conditions, in contrast to ARF8 OX that displayed short hypocotyls in the light. Stronger and weaker apical dominance, and promotion and inhibition of lateral root formation were observed in arf8-1 and ARF8 OX respectively. Sensitivity to auxin was unaltered in arf8-1 hypocotyls with respect to growth inhibition caused by exogenously applied auxin and growth promotion induced by higher temperatures. ARF8 expression was observed constitutively in shoot and root apexes, and was induced in the light condition in hypocotyls. Free IAA contents were approximately 30% reduced in light-grown hypocotyls of ARF8 OX, but were similar between those of arf8-1 and wild type. Expression of the three GH3 genes was reduced in arf8-1 and increased in ARF8 OX, indicating that they are targets of ARF8 transcriptional control. Because the three GH3 proteins may be involved in the conjugation of IAA as suggested by Staswick et al. (2002), and because two of the three GH3 genes are auxin inducible, ARF8 may control the free IAA level in a negative feedback fashion by regulating GH3 gene expression. ARF family genes seem to control both auxin sensitivity and homeostasis in Arabidopsis.

MicroRNA-Directed Regulation of ArabidopsisAUXIN RESPONSE FACTOR17Is Essential for Proper Development and Modulates Expression of Early Auxin Response Genes

“…Only a subset of GH3-like transcripts appears to be regulated by both ARF17 and ARF8; DFL2/GH3.10 mRNA levels are unchanged in 5mARF17 rosette leaves ( Figure 8A), and GH3.3, DFL2/GH3.10, and JAR1/ FIN219 mRNA levels are unchanged in arf8 mutants and ARF8 overexpressing plants (Tian et al, 2004). Intriguingly, ARF8 is regulated by miR167 Kasschau et al, 2003), a miRNA unrelated in sequence to miR160.…”

“…Indeed, we observe expanded DR5-GUS expression domains not only in rosette leaves, but also in 5mARF17 cotyledons ( Figures 8D and 8E), and the changes visualized by histochemical staining are obvious, whereas the changes in GUS mRNA levels are modest (approximately twofold). miR160 and miR167 May Coordinately Modulate GH3-Like Expression ARF8 appears to negatively regulate free IAA levels by controlling GH3-like gene expression (Tian et al, 2004). Levels of three GH3 mRNAs, GH3.5,DFL1/GH3.6,and GH3.17, are reduced in arf8 loss-of-function mutants and increased in ARF8 overexpressing plants (Tian et al, 2004), suggesting that ARF8 activates GH3-like Primer expression.…”

“…miR160 and miR167 May Coordinately Modulate GH3-Like Expression ARF8 appears to negatively regulate free IAA levels by controlling GH3-like gene expression (Tian et al, 2004). Levels of three GH3 mRNAs, GH3.5,DFL1/GH3.6,and GH3.17, are reduced in arf8 loss-of-function mutants and increased in ARF8 overexpressing plants (Tian et al, 2004), suggesting that ARF8 activates GH3-like Primer expression. We found that miR160-directed regulation of ARF17 is also important for proper GH3-like expression (Figures 7 and 8).…”

Bimodular auxin response controls organogenesis in Arabidopsis

“…Our results demonstrate that the control exerted by auxin on lateral root initiation is at least bimodal and consists of the crucial early SLR/IAA14-ARF7-ARF19-dependent auxin response module (3-7, 23, 24), followed by a second BDL/ IAA12-MP/ARF5-dependent module. Most likely, more modules are involved, because other ARFs have been implicated in adventitious and lateral root development (3,(29)(30)(31)(32)(33)(34). We propose that discrete auxin response modules successively coordinate distinct developmental processes, most probably through the regulation of unique targets, comparable to the spatially distinct, bipartite auxin response during hypophysis specification (19).…”

Fine tuning of auxin signaling by miRNAs

“…miR160-regulated ARF17 acts directly to repress GH3.5 and DFL1/GH3.6 transcripts leading to increased free IAA levels. Similarly miR167 guides the cleavage of ARF8 transcript, which apparently negatively regulates free IAA levels by controlling GH3-like gene expression (Tian et al, 2004). Although, the exact mechanism of GH3-like gene regulation by ARF is not very clear, nonetheless it is indicative of an intricate interaction between auxin levels and miRNAs during plant development (Fig 2).…”